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The lock only ensures that initialization is done once. This does not
require a lock, and actually would not require to be done only once at
all since the initialization is idempotent.

This is part of a larger PartitionAlloc refactoring (see linked bug).

Bug: 1061437, 787153
Change-Id: Ie2898b59114216ab76050c47e6d998051d3f3998
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144138Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758198}